The present invention relates to a metering pump, made of plastic, comprising two rotors, which are coupled to one another via gearwheels and are drivable in opposite directions and which are mounted in a pump housing provided with suction ports and outlet ports, wherein each rotor has a rotor shaft, the rotor shaft ends of which are seated in the walls of the pump housing.
Metering pumps are known in all sizes and construction types. As plastics metering pumps are known, in particular, manually actuated piston pumps, as are known on soap dispensers for liquid soaps or, as here particularly of interest, also in the hotel and catering industry, where in fast food establishments, for instance, mustard, ketchup or even coffee cream are delivered in metered doses with manually operated piston pumps of this kind Despite these metering pumps, the delivered quantity nevertheless varies relatively, since, in the metering pumps, in particular of the kind as here just described, the stroke length should actually be fully utilized with each actuation, though this is not generally the case. Instead, one, two or three short strokes are often made and, accordingly, the quantity varies very strongly. As long as this quantity is delivered merely as accompaniment to a hamburger, this is only of minor importance. Where such metering pumps are also used to add a specific quantity of a liquid food to a recipe, however, the taste is varied by incorrect actuation, which is not always appreciated by the customers.
Although a variety of different pumps are certainly known, in particular including rotor pumps, these are generally designed as relatively high-precision metering pumps, made of metal, and this is also necessary in the food industry, where large quantities must be delivered in metered doses. For commercial application, normally very cheap disposable metering pumps are delivered, however, generally free of charge. Accordingly, such metering pumps must be made of plastic, have a simplest possible construction and work reliably.
The plastics metering pump which is here of interest is meant to be designed, in particular, for foods which are delivered in so-called tubular bags or other flexible packagings made of plastics sheets.
Many liquid foods also contain considerable proportions of solids. Typical examples of liquid foods of this kind are, for instance, tartare sauce, mustard sauces with pickles, custard with chocolate or with almond slivers, etc. With the currently standard metering pumps, solid-liquid foods of this kind cannot be delivered in metered doses. In so-called gear pumps, in particular, of the kind which is represented, for instance, in FR-2313971, this is barely realizable. In the case of larger solid particles, such as, for instance, almond sticks, these are ground by the rotors or jam the rotors. Accordingly, for such metering pumps, in particular metering pumps in which the rotors have two or multibladed rolling elements can be considered. Examples of such pumps are known from U.S. Pat. No. 3,054,417, where a metering pump for liquid mediums for the admixture of further liquids is shown, wherein each rotor here has three blade arms and these blade arms roll one against another and thus advance the medium. In such pumps, there is sufficient space between the housing and the individual rotor blades to convey also liquids which contain solid parts. Here, the larger solid parts are less of a problem than, in fact, the smaller solid parts, which remain stuck on the rotor blades which mutually roll one against another and in the rolling operation are completely crushed, whereupon a coating can form which reduces the pumping capacity and can even lead to blockages.
The same also applies to a metering pump according to WO 95/24556, in which merely twin-bladed rotors are represented, which likewise, however, roll mutually both one against the other and against the housing wall.